Any bare metal of your case. TBH you don't need the strap at all. You should get in the habit of touching your case before you touch any components to discharge yourself. Since you already have the strap ordered the hdd bay is probably a good place to hook it so it won't get in the way.
Hello... Something that is a metal frame that is plugged into a wall socket ground/three prong Plug... your electrical system in your house is connected to Earth ground... Also your water pipes are.
Your case is NOT Grounded, unless you have your Power Supply installed/screwed to it, and plugged into the wall, with a 3 prong plug.
Your case will be grounded through your PSU once it is installed and connected to the grounded wall plug. That being said, you do NOT want to plug in your PSU until after you have built the PC. So you will need to connect you wrist strap to some other grounded connection. At any rate that all comes back to connecting it to the ground pin of the wall plug.
Hello... a common Repair Station will use a metal 120v Outlet cover, and attach metal to it for the wrist quick connection.
You gather Static Charge by walking over carpet and floors with socks or shoes... Touch a Grounded Metal Surface before touching parts, Don't walk around with parts in your hand... Keep parts in their packaging when transporting them to your work place.